1. Field of the Invention
This invention concerns a method of controlling pitch in pulps for use in making paper. More particularly, this invention concerns a method of controlling the buildup of pitch on equipment used for pulping and the improved dispersion of pitch in pulps.
2. Brief Description of the Related Art
Pitch, carried over from wood or bark pulped to make papermaking furnishes, is formed of a complex mixture of oleophilic, water insoluble, low molecular weight, essentially non-polar resins comprising fatty and resin acids with ionisable hydrophillic groups, fatty and resin acid esters, sterols, di and tri-glycerides, terpenes and waxes and various alcohols, hydrocarbons and neutral compounds associated with these resins. The resins tend to agglomerate to form sticky films or pitch balls, which in turn can give rise to spotting in the final paper product, wire spots, localised sticky spots on rolls, holes in the paper sheet, poor paper formation, felt plugging or sticking on dryer and calender rolls.
Effective pitch control, particularly in paper mills which produce paper from groundwood, thermomechanical and semi-chemical pulps, has traditionally been difficult. The important parameters for pitch control are elegantly summarised in U.S. Pat. No. 4,964,955 as follows:
"To control pitch effectively it is not necessary to remove all of the pitch from the pulp. Different types of pulp mills have different tolerance levels for pitch particles which must be exceeded before pitch caused problems occur. What is necessary, however, is that the pitch be dispersed as tiny non-harmful aggregated or agglomerated particles and retained in this fine particulateform rather than as larger harmful globs in the paper sheet. Simply flocculating pitch will not give good pitch control; the pitch must be made to attach to either the cellulosic fibres or to any particulate matter used as filler in the paper sheet, or preferably to both. Pitch retained in large flocs causes breaks and undesirable dark specks in the finished paper sheet, while pitch not retained but in small flocs tends to accumulate in the papermaking facility's white water system".
Efforts to control pitch deposit problems by dispersing or precipitating the pitch have traditionally involved the use of alum salts; ultrafine crystalline talc; anionic, cationic and non-ionic dispersants, polymeric retention aids and mixtures of these materials. More recently, water soluble zirconium salt complexes and organotitanium chemicals have been suggested as pitch control agents.
Papermakers alum (Al.sub.2 (SO.sub.4).sub.3 -14H.sub.2 O), liquid sodium aluminate (20-25.5% Al.sub.2 O.sub.3) and polyaluminium chloride are the primary alumina sources used for pitch control. Above pH 3 hydrolysis of the salts occur and a variety of hydroxy-aluminium complexes are formed depending upon the hydroxide ion concentration. Hydroxyl bridging occurs forming dimers and higher order polymer species (ligands). The formation of these desirable short chain relatively low molecular weight polymer species is primarily influenced by the system pH, temperature (the higher the better), the concentration of aluminium ions and the cationic demand of the stock. In pitch control, the optimum pH is preferably around 4.5 although in practice an operating range of 4.0-5.5 is fairly typical. It is believed that pitch control with aluminium salts is a combination of a flocculation mechanism, where the pitch particles are entrapped by the polymeric aluminostructure, and the formation of a cationic complex with the resin and fatty acid components of the pitch, which in turn is then adsorbed onto the pulp fibres. A sodium aluminate/alum combination is generally preferred because there is no need for caustic soda to be used to control pH, it minimises sulphate ion build up in closed white water systems and it limits iron contamination thus making it easier to meet brightness specifications. Typical consumption figures are: alum 5-25 Kg/tonne and 45% sodium aluminate 5-10 Kg/tonne.
Ultrafine talc with a surface area of from 10 to 15 m.sup.2 /gm has been used for pitch control in pulp paper mills and in particular in those mills that are processing Kraft or sulphite pulps, i.e. where the pitch is present in a partially saponified state. The dispersed pitch particles are adsorbed on to the hydrophobic talc platelets and then retained in the paper. Typically, a use rate of 10-40 Kg/tonne is required.
A wide range of surfactants and short chain polymeric dispersants have been used to maintain pitch in a dispersed state within the paper making circuit. These dispersants function by charge and/or steric hinderance mechanisms. Anionic dispersants, such as lignin sulphonates, polynaphthalene sulphonates and polyacrylates, increase the electronegative charge on the pitch particles, i.e. they peptize the pitch particles and inhibit agglomeration. Nonionic surfactants result in steric repulsion of the pitch and depending on the type and concentration can soften and emulsify pitch deposits already formed. Since dispersants do not attach the pitch to the pulp fibres, they have limited use in controlling severe pitch problems such as is generated in TMP mills. An excess of dispersant can result in a substantial build up in the concentration of pitch in closed white water systems. Usage rates for pitch dispersants are typically in the range of 5-100 ppm on the stock.
The use of polyquaternary ammonium polymers for pitch control has been promoted in recent years. The control mechanism is described by T. Hassler, Tappi Journal, June 1988, p 195 onwards. The low molecular weight cationic polymers which are commercially available are believed to be linear and/or partially crosslinked polydiallyldimethyl ammonium chloride (polyDADMAC) and copolymers of epichlorohydrin and short chain aliphatic secondary amines. A typical use rate would be from 1.25-2.3 Kg/tonne.
The use of current retention and drainage aid technologies e.g. "Lapotain" (Laporte Industries Limited), "Composil" (Eka-Nobel) and -Positek- (Nalco Chemical Co) etc, give significant improvements in pitch control on the machines, but reliance on this type of technology however is only possible when the mill can justify a retention and drainage aid programme.
More recently, a method of reducing pitch in pulping and papermaking operations by a process which comprises (1) adding to a pitch-containing furnish a particulate composite substance comprising (a) a water soluble cationic polymer adsorbed onto (b) an essentially water insoluble particulate substrate carrying an anionic charge, and (2) adsorbing pitch onto said composite to form discrete. finely dispersed pitch-containg aggregates in the furnish, is disclosed in U.S. Pat. No. 4,964,955. On Column 8, line 67, to Column 9, line 15, of the U.S. patent, it is disclosed that any essentially water-insoluble particulate organic or inorganic substance may be employed as the substrate, including phyllosilicate minerals, kaolin, talc, mica, montmorillonite, chlorite and pseudolayer silicates, though Kaolin is particularly preferred. Indeed, this preferrence is clearly demonstrated in the examples in that Kaolin is the only substrate exemplified. It is demonstrated in Examples IV to VI, IX and X that improved pitch control may be achieved by pretreating a groundwood pulp with alum prior to treatment with a cationic kaolin. In Example XII, on Column 19, lines 12 to 16, it is disclosed that using 60-80 lbs/ton of cationic kaolin slurry with 35 lbs/ton of alum will essentially eliminate all pitch. From a practical point of view, however, the amount of cationic kaolin required in the process tends to make a papermaking process uneconomic. A similar method is disclosed in EP-A-0349311.
Japanese patent application No 62-245908 discloses a method for reinforcing paper strength by adding bentonite to a pulp after a cationic paper strength reinforcing agent has been mixed in the pulp. It is noted in the patent application that the process, apart from improving the paper strength, also solves pitch troubles. It is disclosed in Example 1 and Example 2 that a minimum of 10 Kg/tonne (1% relative to pulp) of bentonite was added to a pulp which had been previously treated with reinforcing agent and alum. Whilst this method appears suitable for improving paper strength, retention and pitch problems, as a method for controlling pitch alone, this process would tend to be uneconomic.
It is an object of the present invention to provide a process for controlling pitch in pulps, which process is technically at least as efficient at eliminating or reducing pitch in pulps as the process described in U.S. Pat. No. 4,964,955 but which is economically more acceptable.